1. Field of the Invention
This invention relates generally to liquid crystal display (LCD) devices, and more particularly to a method of manufacturing LCD devices. Even more particularly the present invention relates to manufacturing processes and the LCD display devices manufactured thereby that decrease the incidence of defects arising over time, including the catastrophic failure of liquid crystal on silicon (LCoS) light valves. Furthermore, the present invention also relates to LCD display devices and manufacturing processes that increase the yield of LCD display devices from a single wafer.
2. Description of the Background Art
Reflective and transmissive liquid crystal display (LCD) devices are used in video projectors, rear projection televisions, computer displays, and so on as a means for producing high quality imagery. Known LCD devices have decreased the size, weight, and overall cost of many electronic products, while at the same time increased the quality of imagery produced when compared to conventional alternatives such as cathode ray tubes (CRT).
FIG. 1 shows a reflective liquid crystal on silicon (LCoS) light valve 100, which is a known type of LCD device. Display device 100 is formed on a silicon substrate 102, and includes an integrated circuitry layer 104, insulating layer 106, a plurality of pixel mirrors 108, a planarized layer 110, a protective coating 112, a lower liquid crystal alignment layer 114, a liquid crystal layer 116, an upper liquid crystal alignment layer 118, transparent electrode layer 120, a transparent (e.g., glass) substrate 122, an anti-reflective coating 124, and a gasket 126. The thicknesses of the layers depicted in FIG. 1 are not shown to scale, but are instead exaggerated so as to be more clearly visible.
Mirrors 108 are coupled to circuitry layer 104 through a plurality of vias formed in insulating layer 106. Planar layer 110 and protective layer 112 provide a flat, relatively robust surface for subsequent layers of the device. The thickness of planar layer 110 and protective layer 112 over mirrors 108 are on the order of optical thin film coatings. Alignment layers 114 and 118 help to properly align the liquid crystals of layer 116. Transparent electrode 120 (e.g., Indium Tin Oxide) is formed on the bottom surface of glass substrate 122, and anti-reflective coating 124 is formed on the top surface. Alignment layer 118 is formed on transparent electrode 120.
During operation, light passes through all upper layers 124, 122, 120, 118, 116, 114, 112, and 110 of device 100 to impinge on pixel mirrors 108, is reflected from the top surfaces of mirrors 108, and then exits the device again passing through upper layers 110, 112, 114, 116, 118, 120, 122, and 124. The polarization of the light is altered by liquid crystal layer 116, depending on the electrical field across liquid crystal layer 116. When transparent electrode 120 is held at a particular voltage, the electrical field across liquid crystal layer 116 is controlled by the voltages asserted on pixel mirrors 108 by circuitry layer 104. Thus, the polarization of spatially pixilated portions of the incident light can be individually modulated.
Alignment layers 114 and 118 provide a means of aligning the nematic liquid crystals of liquid crystal layer 116. This alignment is accomplished by inducing a topographical asymmetry in the surface. The surface asymmetry causes the liquid crystal molecules to be pinned at the surface. Consequently the bulk orientation of the LC is controlled by the surface orientation.
One known method for forming alignment layers includes forming a polyimide layer and then mechanically rubbing the polyimide layer in a predetermined direction to create the surface asymmetry. One common limitation of polyimide alignment layers is that they are not very stable under high intensity illumination.
To address the limitations of polyimide alignment layers, evaporated thin film alignment layers were developed. These evaporated thin film layers are typically formed from oblique evaporation of silicon oxide (SiO) or silica (SiO2). The evaporated thin film layers have been found to create very stable alignments under high intensity illumination. This is very important for consumer TV applications and so it is the preferred method of alignment for LCOS displays.
A problem has arisen, however, with devices incorporating evaporated thin film alignment layers. In particular, the pretilt angle (a property of the liquid crystal material in the liquid crystal layer) is observed to change over time, thereby changing the performance of the display. This change affects both the contrast and color of the projected image and so is deemed unacceptable for consumer applications. The gradual failure of LCD devices appearing over time can be particularly costly and bothersome to manufacturers because the devices will most likely have been incorporated into other products by the time the defects become apparent. Then, failure of the device can result in diminished optical performance of the product, costly warranty repairs, and/or costly product recalls.
Gasket 126 contains and seals the liquid crystal layer 116. Gasket 126 is formed between alignment layers 114 and 118 and around the liquid crystal layer 116. In particular, gasket 126 surrounds the active pixel area of LCD device 100. Despite the sealing properties of gasket 126, LCD device 100 will still experience performance degradations over time.
Another problem associated with gasket 126 is that in prior art devices gasket 126 must be relatively wide and takes up a large amount of area on LCD device 100. Accordingly, LCD device 100 must be made large enough to accommodate gasket 100 around its active pixel area. For example, prior art gaskets, like gasket 126, are manufactured to a width (w) of 800 micrometers (0.0315 inches) or greater. Indeed, gasket widths greater than 1000 micrometers are common. As a result, up to 50% of the area of the LCD device can be devoted to gasket contact area. This in turn significantly reduces the number of display devices that can be produced from a single silicon wafer.
What is needed, therefore, is a method of manufacturing an LCD device that is less likely to have initial image defects and less prone to future failure. What is also needed is a method for forming an alignment layer in an LCD device that does not adversely affect device yield or useful life. What is also needed is an LCD device that is less prone to future failure than known devices. What is also needed is an LCD device and method of manufacturing the same that results in a greater yield of LCD devices per wafer.